The overall aim of this project is to define the maternal contribution to early mammalian development. The goal is to characterize the RNA components of the mouse egg, to examine their synthesis during oocyte development, their storage in the growing oocyte, and their role in early embryogenesis. Current studies include the analysis of the retention of labeled maternal RNA in early embryos from the 2-cell to the blastocyst stages. The distribution of grains between the inner cell mass and the trophoblast, and between nucleus and cytoplasm will be examined. The synthesis of total chromosomal RNA in growing mouse oocytes has been studied by labeling oocytes in an in vitro culture system known to permit oocyte development. The kinetics of labeling of heterogeneous RNA relative to that of ribosomal RNA has been used to calculate that the rate of synthesis of heterogeneous RNA is high, but still much lower than expected for active lampbrush chromosomes. Cytological studies tend to confirm this finding. Experiments are underway to measure the absolute amounts of rRNA accumulating during oocyte growth. The amount of rRNA in samples prepared from oocytes of different sizes will be measured by the 'titration' reaction with excess labeled cDNA complementary to rRNA.